In the bowl of a mixer, cream together butter and sugar. Add in eggs, sour cream, and vanilla extract. Stir to combine.
Sift together the flour, baking powder, salt, and baking soda. Mix dry ingredients into butter and sugar.
Form dough into a disc shape, place in a Ziploc bag and chill dough overnight in the fridge. (I have made this dough before without chilling overnight, just stick the dough in the freezer for 30 minutes to an hour to chill.)
Once chilled, roll the dough out between two piece of parchment paper (or on a lightly floured surface). Make sure the thickness is somewhere between 1/4 to 1/2 inch thick.
Once rolled, cut out shapes and place onto a baking sheet. Continue to gently reform, roll, and cut the dough until all of it is used.
Bake at 350°F for 10-12 minutes, or until edges become golden brown. (Bake time greatly varies based on shape and size of cookie. Pay attention and remove the cookies as soon as the edges start turning brown. The cookie will set as it cools).
Once the cookies are baked, let them rest on the baking sheet for 2-3 minutes before removing to a cooling rack. Let cookies cool completely, before decorating.
In the bowl of a mixer, combine the meringue powder and water with the paddle attachment. Beat until mixture is combined and starting to foam.
Add in the sifted powdered sugar, stir to combine. Make sure to scrape the sides and paddle with a spatula, if necessary.
Next, stir in the corn syrup and vanilla extract.
Now increase the speed of the mixer to medium-high. Beat the icing for about 5 minutes, until the icing has formed stiff peaks.
This icing consistency is "Home Base". White and the correct thickness for stiff boarders and hard details. Press a piece of plastic wrap onto the surface of the icing while you make colors and flood icing.
Take a large spoonful of the basic royal icing and place into a small bowl. Stir in a drop or few drops of gel food coloring to the icing until it reaches your desired shade.
The basic royal icing consistency is too thick to fill in color on the whole cookie. I decorate all my cookies at a "flood" consistency. Which is just thinned icing. To your colored royal icing from above, add a teaspoon (or half a teaspoon) of water at a time. Stirring after each teaspoon until the icing can be "drizzled" in a ribbon pattern into the bowl and "disappear" back into the surface of the remaining icing within 2 seconds (counting one thousand one, one thousand two). Anything stiffer than that will not flood and anything thinner risks the icing flooding right off the cookie onto your counter.
Pour thinned and colored icing into a small plastic squeeze bottle and enjoy decorating your Christmas Sugar Cookies!